Hydrogen generators are devices that generate hydrogen gas for use in fuel cells, combustion engines, and other devices, often through the evolution of hydrogen gas from chemical hydrides, borohydrides or boranes. Sodium borohydride (NaBH4) has emerged as a particularly desirable chemical hydride for use in such devices, due to the molar equivalents of hydrogen it generates (see EQUATION 1 below), the relatively low mass of NaBH4 as compared to some competing materials, and the controllability of the hydrogen evolution reaction:NaBH4+2H2ONaBO2+4H2  (EQUATION 1)
The hydrolysis of hydrogen-generating materials in general, and sodium borohydride in particular, as a method of hydrogen generation has received significant interest in the art, due to the high gravimetric storage density of hydrogen in these materials and the ease of creating a pure hydrogen stream from the hydrolysis reaction. However, in some applications, such as when hydrogen generators are used in combination with hydrogen fuel cells to power laptops or handheld devices and electronics, the inability to adequately control the generation of hydrogen gas is a drawback from a system perspective. Ideally, in such an application, the hydrogen generator should be able to produce a stream of hydrogen gas promptly when the gas stream is needed, and should likewise be able to promptly terminate the flow of hydrogen gas when it is no longer needed.
In reality, however, most hydrogen generators currently available display a significant lag time from the point of time at which the demand for hydrogen commences, and the point of time at which the flow of hydrogen gas is suitable to meet that demand. Perhaps more significantly, the flow of hydrogen in most currently available hydrogen generators does not cease with demand, and may even proceed until the hydrogen generating material has been depleted. The generation of hydrogen gas in excess of demand is problematic for hydrogen generators in general, and for small hydrogen generators (of the type designed for incorporation into laptop PCs and hand-held devices) in particular. Aside from the danger of fire or explosion, the excess gas creates pressure spikes that can damage the generator and its components.
Moreover, the need to accommodate such pressure spikes and to store excess hydrogen gas requires hydrogen generators to be heavier, bulkier, stronger, and more complicated than would otherwise be the case. Since space and weight are typically at a premium in laptop computers and hand-held devices, this is a serious drawback in hydrogen generators. Venting excess hydrogen gas is typically not an option in these devices due to the obvious fire risks and, in any event, is undesirable in that it reduces the effective yield of the hydrogen generator.
There is thus a need in the art for a hydrogen generator that offers fast response time to the need for hydrogen so that a supply of hydrogen is available on demand. There is also a need in the art for a hydrogen generator that effectively halts the production of hydrogen gas when the demand for hydrogen abates, so that excess hydrogen is not generated. There is further a need in the art for a hydrogen generator with minimum dimensions, weight and space requirements. These and other needs are met by the devices and methodologies disclosed herein.